This invention relates generally to an improved touch control circuit arrangement for electrical appliances of the type having one or more electrical loads to be individually controlled in accordance with user selected inputs, such as, for example, domestic electric range having a plurality of surface heating units for cooking.
Electronic touch control panels enabling the user to select a variety of operating modes and power settings as well as ON and OFF are well known particularly in appliances such as microwave ovens. Typically, the touch panel involves a matrix array of touch switch members or keys having a plurality of input lines enabled by signals from the microprocessor control and output lines to couple the enable signal back to the microprocessor for sensing an actuated key. In such arrangements user inputs including ON/OFF selections are multiplexed into the microprocessor and require the microprocessor to be operating properly in order for the user inputs to be implemented.
It is highly desirable that the user be able to turn off the appliance; that is, interrupt or prevent energization of the power circuitry even if there is a microprocessor failure. Microwave ovens are typically equipped with a safety door interlock switch arrangement which operates to interrupt energization of the power supply when the door is open. Such circuits work independently of the microprocessor and hence inherently enable the user to turn off the appliance independently of the microprocessor by simply opening the door. Range cooktops, however, do not have such an interlock arrangement for the surface units. Consequently, some means is needed to enable the user to turn off the surface units even in the event of a microprocessor failure. One solution would be to provide dedicated ON/OFF circuits for each of the surface units of the appliance. The cost and complexity of the duplicate circuitry renders this approach undesirable. It would be desirable therefore to provide a control circuit which retains the advantages of the multiplexed keyboard arrangement and which also enables the user to turn off the power circuitry independently of the microprocessor.
It is, therefore, an object of the present invention to provide an improved touch control circuit for an appliance in which user inputs are provided to the microprocessor controller in multiplex fashion thereby avoiding the cost and complexity of duplicate ON and OFF circuitry while directly coupling the OFF key for each surface unit directly to the power control circuitry to enable the user to turn off the appliance independently of the microprocessor.